Ceramic composites are gaining emphasis in diverse applications such as heat engine components, cutting tools and various wear resistant parts. The ceramic composites typically have improved fracture toughness and improved wear properties. Conventional ceramics are generally monolithic materials and have low fracture toughness. This makes these materials brittle and they are liable to crack under stressed conditions, and are not very useful for diverse demanding industrial applications. Monolithic ceramic materials such as silicon carbide, alumina, silicon nitride and mullite have low fracture toughness (K.sub.IC) of between 2.5 to 4.5 Mpa.sqroot.m).
There has been extensive research underway to produce ceramic composites of higher fracture toughness, using a matrix such as alumina, silicon nitride and silicon carbide, reinforced by materials such as silicon carbide particles which may be in the form of, for example, whiskers or fibers.
Alumina as a matrix material with silicon carbide whisker reinforcement for ceramic composites has received strong attention. Fracture toughness and strength of alumina-silicon carbide whisker composites (Al.sub.2 O.sub.3 --SiC) are much higher than monolithic alumina. The improved strength and fracture toughness are retained to high temperatures of around 1200.degree. C. The reports on SiC whisker reinforced alumina matrix composites have been so encouraging that the product is already being commercially produced as cutting tools, wear dies and in other applications. Cutting tool materials typically contains around 30% SiC whiskers and show much better resistance to wear and fracture in machining operations. It has even been reported by E. R. Biollman, P. K. Mehrotra et al. Am. Cer. Soc. Bull., 1988, 67, 1016, that the estimated savings in field tests of machining a superalloy is 73% with the Al.sub.2 O.sub.3 --SiC composite tools as compared to machining with just Al.sub.2 O.sub.3 tools.
Typically these composites (G. C. Wei, U.S. Pat. No. Re. 32,843) are produced by mechanically mixing single crystal SiC whiskers with fine ceramic powders such as alumina (Al.sub.2 O.sub.3) such that the SiC whiskers are uniformly and homogeneously dispersed. This homogeneous dispersion is normally difficult to achieve through mechanical mixing processes. The mixture is then densified through techniques such as hot pressing at pressures in the range of 28 to 70 MPa and temperatures in the range of about 1600.degree. to 1950.degree. C. with pressing times varying about 0.75 to 2.5 hours.
The commercial way of producing silicon carbide is to pass methane gas or the like through silica (SiO.sub.2) in a reducing atmosphere which normally would be formed by the gas that it produces, CO, and reacting the carbon from the methane with silica to reduce it to SiO and then to react SiO with the carbon to form SiC. Various prior art references disclose the conversion of silica (SiO.sub.2) in the presence of a hydrocarbon gas (as the carbon source) to SiC. See, U.S. Pat. No. 4,327,066 to Seimiya, U.S. Pat. No. 4,377,563 to Seimiya and JP 91072008 (SiO). Other prior art references disclose the conversion of various silanes, including halosilanes, alkoxysilanes and alkylsilanes, in the presence of a hydrocarbon to SiC. See U.S. Pat. No. 5,165,916 to Cheng et al., U.S. Pat. No. 5,178,847 to Judin et al.( also citing JP 59102809 and J. Less-Common Metals, 68(1979), pp.29-41), GB 1,134,782, EP 199482, JP 04089357, JP 57175718, JP 04002662, JP 60077114, JP 59131509 and JP 1083510. SiC also is produced from solid silicon which is vaporized in the presence of a hydrocarbon. See. JP 04139014, JP 61097126, JP 56058537, JP 03088709 and JP 63123436.
There are a number of major problems with the above ceramic composite and process for making same. Each of these methods for producing silicon carbide requires that the resulting silicon carbide then be mixed with a suitable matrix, e.g., alumina, to form the desired ceramic composite powder. Further, the silicon carbide whiskers, in particular, are very expensive as they are made primarily through a VLS process. Recently, however, silicon carbide whiskers are being produced from rice hull, which is a cheap raw material, reducing the cost of whisker production. A further concern relates to the fact that the silicon carbide whiskers are carcinogenic and are very dangerous to handle. This procedure has the attendant deficiencies and difficulties associated with producing a homogeneous mixture of these components. The dispersion of very fine SiC particles, e.g., whiskers, is difficult to achieve and elaborate processing techniques are necessary. With mechanical methods of mixing whiskers and ceramic matrix powders, there is the possibility of whiskers clustering together and whisker damage, and the extent of whisker loading is limited. For example in order to obtain a good dispersibility of whiskers and thus improve the strength of the composite, both ultrasonic dispersion techniques and finer particle, non-agglomerated matrix powder have to be used (P. F. Becher and G. C. Wei, Journal of the American Ceramic Society, 1084, 67,C267). Very elaborate processing techniques, involving flotation or sedimentation from dispersions of the components, were found to be effective in eliminating the potential flaw types (J. Homery, W. L. Vaughn and M. K. Ferber in the American Ceramic Society Bulletin. 1987, 67,333). However, with the information that the SiC whiskers are very carcinogenic, all these complex processing techniques have become very unattractive.
Since it is difficult to produce a uniform dispersion and provide fine particles by a powder mixing method, in situ SiC production is desirable. Along this line, a method utilizing thermal decomposition of organometallic macromolecular substances has been developed. When organometallic macromolecular substances which include metal elements for forming ceramics, such as silicon, are subjected to thermal decomposition in an inert nitrogen-containing atmosphere, organic components are removed and carbides, nitrides, carbonitrides, oxynitrides, etc., and mixtures thereof can be obtained. The organometallic macromolecular substances together with carbon fibers are dispersed in a mullite matrix. The fine ceramic particles produced as a result of the thermal decomposition of such substances form a boundary layer surrounding the grains of mullite and carbon fibers. See U.S. Pat. No. 5,077,243 to Nakano, et al.
In another in situ SiC production scheme, Al.sub.2 O.sub.3 and SiC composite materials have been produced from mixtures of alumino-silicates and carbon (solid) as precursor materials. These composites are produced by heating a mixture of an alumino-silicate, e.g. kaolinite, and carbon so that only Al.sub.2 O.sub.3 and SiC remain as major phases. U.S. Pat. No. 5,011,799 and U.S. Pat. No. 5,096,858 to Das Chaklader, et al. and Das Chaklader, et. al., "Al.sub.2 O.sub.3 -SiC Composites from Aluminosilicate Precursors," J. Am. Ceram. Soc., Vol. 75, No. 8, pp. 2283-85 (1992). I. Higgins, et al. produced .beta.'-sialon by the carbothermal reduction in a nitrogen atmosphere of kaolinite, which had a carbon source, e.g., carbon black or coal, mixed therein. When argon was substituted for nitrogen, .alpha.-Al.sub.2 O.sub.3 and .beta.-SiC were formed instead.
The method of present invention overcomes the difficulties and deficiencies of the prior art which separately produces SiC and then incorporates it into alumina in that the SiC is formed in situ obviating the separate mixing step of the prior art and associated handling concerns relating to the carcinogenic effects of SiC whiskers. Further, the SiC is obtained in situ without any interference from the presence of the alumina component of the alumino-silicate. Though the previous patents and article to Das Chaklader et al. also involve the in situ formation of SiC in an alumino-silicate, a solid source of carbon is used therein and requires that it be homogeneously mixed with the alumino-silicate. In this sense, this method suffers from the same problem of the prior art which separately produced SiC, i.e., a homogeneous mixing step. On the other hand, the present invention utilizes a carbon source which is not normally solid when introduced into the reactor and does not require the prior addition and mixing of solid carbon particles in the alumino-silicates to provide a homogeneous combination of alumina (Al.sub.2 O.sub.3) and silicon carbide (SiC) in the final powder. However, in the present invention, carbon particles having a particular size and shape still may be added into the reactor in order to control the morphology (i.e., the size and shape) of the carbide phase and this would require a mixing step.
Though an in situ production scheme is used by Nakano, the present invention is also distinguishable therefrom in that the methods and starting and ending materials are different. Nakano also requires a homogeneous mixing step. Further, the organometallic macromolecular substances required therein are much more complicated and presumably more difficult and costly to produce or obtain than using a gaseous carbon source, e.g., a hydrocarbon, and an alumino-silicate as in the present invention.